This invention relates to a double bounce passive facet tracking system and more particularly to a method which uses a reflective diffraction grating on a flat facet of a rotating polygon assembly instead of a dual mirror rotating polygon assembly using convex and flat mirrors.
The propagation of a light beam can be changed by three basic means: reflection by a mirror, refraction by a lens and diffraction by a grating. Optical systems traditionally rely on reflection and refraction to achieve the desired optical transformation. Optical design, based on mirror and lens elements, is a well established and refined process. Until recently, the problems with diffraction and fabricating high efficiency diffractive elements have made diffractive elements infeasible components of optical systems.
The diffractive process does not simply redirect a light beam. Diffraction, unlike refraction and reflection, splits a light beam into many beams--each of which is redirected at a different angle or order. The percentage of the incident light redirected by the desired angle is referred to as the diffraction efficiency. The diffraction efficiency of a diffractive element is determined by the element's surface profile. If the light that is not redirected by the desired angle is substantial, the result will be an intolerable amount of scatter in the image or output plane of the optical system.
Theoretically, on axis diffractive phase elements can achieve 100 percent diffraction efficiency. To achieve this efficiency, however, a continuous phase surface profile is necessary. The theoretical diffraction efficiency of this surface profile is also relatively sensitive to a change in wavelength. By contrast, refractive elements are wavelength insensitive, with respect to throughput, over most of the transmission band.
However, simple diffractive elements can be designed for use in systems, such as printing and copying systems, which use a single wavelength of light and require only simple beam changes. One advantage of using simple diffractive optical elements is that diffractive optical elements are generally smaller, thinner and lighter, reducing size and weight of an overall optical system. A second advantage of using simple diffractive optical elements is that they may be fabricated using simple stamping or pressing manufacturing techniques once a master has been made, reducing manufacturing costs.
Conventional facet tracking deflection, as shown in FIG. 1, has been provided through a first bounce of a focused beam on a convex facet 6 residing above (or below, depending on design choice) a flat facet 4 of a dual rotating polygon 2. This convex facet 6 imparts a small angle of deflection needed to the focused beam, i.e. roughly 1.5.times.beam divergence, so that its travel through a dual pass Fourier transform lens will bring it back collimated and centered on the flat facet 4 below from start of scan through end of scan. This configuration is well known in the art. Although currently projected to cost much less than an acoustooptic deflector, the second polygon with the convex facets registered properly with respect to a standard polygon will add mass and size to the overall polygon resulting in, among other things, slower rotation speeds.
It is an object of this invention to provide a double bounce passive facet tracking system and more particularly to provide a reflective diffraction grating on a flat facet of a rotating polygon assembly instead of a dual mirror rotating polygon assembly, using convex and flat mirrors, thereby reducing the weight and size of a rotating polygon assembly.